MOSCOW (Reuters) - The Kremlin complained on Friday that the mingling of so-called moderate rebels with Nusra Front fighters on the ground in Syria made it hard to distinguish between the two when it came to targeting air strikes.

Dmitry Peskov, a Kremlin spokesman, made the remarks when asked to comment on allegations from a senior unnamed U.S. defense official who accused Russian forces in Syria of bombing U.S.-backed rebels.

"Our air force operation is continuing in Syria," Peskov told reporters on a conference call.

"It is not a secret for anyone that the continuing mingling in places of the so-called moderate opposition with Al-Nusra is a really serious problem.

"It really is complicating anti-terrorist action," he said, referring to Russia's campaign of air strikes in Syria.